The present invention relates to a fuel system for injection of a fuel mixture in a combustion engine.
A way of reducing the discharge of emissions from combustion engines is to inject the fuel at a very high pressure. A so-called “common rail” system is widely used to effect injection at a high pressure in the combustion spaces of a diesel engine. A common rail system comprises a high-pressure pump which pumps fuel at a very high pressure to an accumulator tank (common rail). The pressure in the accumulator tank during operation may be of the order of 350-1600 bar. The fuel in the accumulator tank is intended to be distributed to all of the engine's cylinders. The fuel from the accumulator tank is injected into the combustion spaces of the respective cylinders via electronically controlled injection units. The injection units comprise valves which have to be able to open and close very quickly. The injection units are controlled by an electrical control unit which calculates substantially continuously the amount of fuel to be supplied to the respective cylinders on the basis of information about various parameters of the engine, e.g. load and speed.
There is increasing interest in using more than one fuel in combustion engines. It is often of advantage to be able to inject a fuel mixture made up of two fuels which have different characteristics. For example, a less readily ignitable main fuel may be mixed with a more readily ignitable supplementary fuel, and the thermal energy arising from the ignition of the supplementary fuel is used to ignite the less readily ignitable main fuel. This makes it unnecessary to use other types of additives to raise the ignitability of the main fuel.
Injecting two fuels at high pressure in a combustion engine usually involves using two separate fuel supply systems, viz. a first fuel supply system with a first high-pressure pump which pumps the first fuel to a high pressure in a first accumulator tank (common rail), and a second fuel supply system with a second high-pressure pump which pumps the second fuel to a high pressure in a second accumulator tank (common rail). Thereafter the two fuels can be injected at high pressure from the respective accumulator tanks via suitable injection units. US 2002/0070295, incorporated herein by reference, refers to an example of such an injection unit. Using two separate fuel systems involves a large number of components. The two systems occupy a relatively large amount of fitting space and the component cost is also high.